Additional Hydraulic Structures

7/31/2019 Additional Hydraulic Structures

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Sec tion 1100

Ad ditiona l Hydraulic Struc tures1 1 0 1 INTRODUCTION

Presented in this section are design guidelines and standards for hydraulicstructures which are appurtenant to both storm sewer outlet and open channeldesign. These guidelines and standards are generalized since each structureis unique, with the possible exception of channel drops. The user isencouraged to coordinate with the local entity and/or the CCRFCD whenplanning and designing these types of hydraulic structures.

1 1 0 2 CHANNEL DROPS AND ENERGY DISSIPATIONSTRUCTURES

The design of open channels often require the use of channel drop and/orenergy dissipation structures to dissipate excess energy created by gravityacting on the storm water flow. The most common use of these structures isto control the longitudinal slope of channels to keep design velocities withinacceptable limits (Section 700). These structures are also used to dissipateexcess energy at storm sewer outlets and to safely lower flood flow elevationsat abrupt drops in existing topography.

For the purposes of this MANUAL, channel drop and energy dissipationstructures are classified into two groups. Channel drops are classified asstructures which shall only be used when the inflow channel flow is sub-critical

(Froude Number, 0.86). Energy dissipators and stilling basins, areclassified as structures which may be used for either sub-critical (F, 0.86) orsuper-critical (F, 1.13) inflow conditions.

Presented in Table 1101 is a listing of the structures discussed in this sectionalong with the hydraulic limitations under which these structures are allowedto be used within the Clark County area. The designer must obtain priorapproval from the local entity to use any of the listed structures outside of thestated limits. Also, if the designer desires to use a structure not discussed inthe section, pertinent detailed information on said structure must be submittedto the local entity for review and approval prior to designing the facility.

Criteria and charts to aid in the design of these types of structures have beendeveloped based on many various hydraulic studies. Generalized standardsfor each type of channel drop based on these hydraulic studies are providedin the following sections.

The reader should refer to the standard channel drop and energy dissipationdesign references to become familiar with the detailed information available on

Adopted August HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 1102


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Section 1100 Additional Hydraulic Structures

each structure prior to design. Suggested references include Peterka, 1978;USBR, 1987; and 1970.

1102.1 Channel Drop Structures

Presented in Table 1101 are the types of channel drop structures allowed inthe Clark County area. By definition, channel drop structures are to be usedonly when the upstream channel flow is sub-critical. Presented in Figure 1101are the generalized profiles and nomenclature for drop structures. Thisnomenclature is used throughout this section for discussion of specificstandards for each part of the structure. The nomenclature is also applicableto drop structures.

1102.1 Sloping Drop Structures

Presented in Table 1102 and Figure 1102 are the design standards anddetails for sloping drop structures. The design chart for slopingdrop structures is based upon the unit discharge (q) of the approach channel,the classification and the slope of the drop structure, and is valid onlyfor sub-critical flow in the approach channel (i.e., Froude Number (F,) 0.86).The unit discharge is found by taking the average or normal channel velocity(V,) for the discharge times the normal depth of the channel (Y

The design chart is also based upon a prismatic channel section throughout,from the upstream channel through the drop to the downstream channel. Themaximum (steepest) allowable side slope for the lined channel within thedrop structure is Flatter side slopes are allowable and encouraged when

available ROW permits.The classification of chosen for the sloping portion of the structureshould be used throughout the structure, including the upstream anddownstream aprons, the channel bottom and side slopes. See Section 700 for

classification. The should extend up the side slopes to a depthequal to 1 foot above the normal major storm flow depth projected upstreamfrom the downstream channel, or 1 foot above the critical depth in the slopingsection, whichever is greater (see Figure 1102). The maximum fall allowedat any one drop structure is 3 feet from the upper channel bottom to the lowerchannel bottom, excluding the trickle channel.

A detailed description of the drop structure and the design procedureproceeding from upstream to downstream is given below based onFigure 1102.

Criteria

a . Approach Depth: The upstream and downstream channels willnormally be trapezoidal sections with low flow channels to convey



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normal low water flows. The maximum normal depth, is 5 feet andthe maximum normal velocity, V,, is 7 fps.

b. Low Flow Channel: The low flow channel shown in this case is arectangular concrete channel. The concrete channel ends at theupstream end of the upstream apron. A combination cut-off walland foundation wall is provided to give the end of the low flow channeladditional support. The water is allowed to trickle through theupstream apron and through the crest wall (discussed below).low flow channels would simply feather into the upstream apron.

C. Approach Apron: A IO-foot long apron is provided upstream ofthe cutoff wall to protect against the increasing velocities andturbulence which result as the water approaches the sloping portion ofthe drop structure. The same design and bedding should beused as specified for the portion of the drop structure downstream ofthe cut-off wall.

d . Crest Wall: The crest wall is a very important part of the drop structure,and has several purposes, one of which is to provide a level rigidboundary section and distribute the flow evenly over the entire width ofthe structure. This is extremely important since the selection of the

is based upon the unit discharge, and without the wall, flowconcentrations could result which would greatly exceed the designdischarge. The crest wall is also used to reduce or eliminate seepageand piping along with the failures which can result from these problems.

The low flow channel is ended at the upstream end of the upstreamapron to prevent the low flow channel from concentrating additionalwater at a point during high flows, thus exceeding the design unitdischarge. The apron and the crest wall combine to disperse theconcentrated flow. The low flows must be allowed through the crestwall to prevent ponding. A series of notches in the wall will allow thelow flows to do this. The size and number of notches will depend onthe design discharge of the low flow channel. Note that they are offsetfrom the trickle channel to permit flow of water through the upstreamapron. The voids in the below the notch inverts are expected tosilt in rapidly or they can be filled at the time of construction.

The two most common types of walls used will be reinforced concreteor sheet pile. The design of the wall is a structural problem which willnot be addressed here. The depth of the wall should be at least to thebottom of the bedding material and could be deeper if necessary for thecontrol of piping.

The top of the crest wall should be placed a distance P above theupstream channel bottom. This is done to create a higher water



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surface elevation upstream, thus reducing the effectsnormally caused by a drop structure. P can be determined fromTable 1104 and is not considered in the total allowable vertical drop.

e . Chute Apron: The allowable size of and chute slope for thechute portion of the drop structure and the downstream apron areobtained from Table 1102. The size and chute slope aredetermined from the table by first computing q = Next, enter thetable at the proper value of q in the left-hand column. Then, determinethe allowable slopes in the row for that q and select the bestcombination of classification and slope using site and costconsiderations.

The length of the downstream apron and the depth of the can also be obtained from Table 1102. The must be placed onbedding as shown in Figure 1102. The term bedding used in thissection (Section 1100) refers to 6-inch Type II Aggregate Base asspecified in the STANDARD SPECIFICATIONS. The 2 foot long filterfabric cutoffs help prevent piping failures. The should extend upthe side slopes a distance of + 1 foot as projected from thedownstream channel or the critical depth plus 1 foot, whichever isgreater. The side slopes for the chute and downstream apron shouldbe the same as the crest wall and upstream channel with the exceptionthat a slope as steep as can be used starting above theheight of the lining required above. The thickness of theimmediately downstream of the crest wall should be increased toas shown in Table 1102. This extra thickness is necessary to protectthe most critical area of the structure. The voids in the apron can befilled during construction to reduce ponding of low flows in the apronarea.

f. Exit Depth: The downstream channel should be the same as theupstream channel, including a low flow channel. The low flow channelinvert must be below the top of the adjacent section to ensurethat low flows will drain into the low flow channel. For concrete low flowchannels a foundation wall similar to the one used for the upstream lowflow channel should be used. In some instances the wall may also beused to control seepage and piping.

A design example for a sloping drop structure is presented inSection 1102.3.1.

1102.1.2 Vertical Drop Structures

Presented in Table 1103 and on Figure 1103 are the design standards anddetails for vertical drop structures.



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The design chart for the vertical channel drop structures is based upon theheight of the drop and the normal depth and velocity of the approach and exitchannels. The channel must be prismatic throughout, from the upstreamchannel through the drop to the downstream channel.

The maximum (steepest) allowable side slope for the stilling basin isFlatter side slopes are allowable and encouraged when available ROW

permits. The should extend up the side slopes to a depth equal to 1 footabove the normal depth projected upstream from the downstream channel.(See Figure 1103). The maximum fall allowed at any one drop structure is3 feet from the upper channel bottom to the lower channel bottom, excludingthe low flow channel.

A detailed description of the drop structure and the design procedure fromupstream to downstream is given below and is presented on Figure 1103.

a . Approach Depth: The upstream and downstream channels willnormally be trapezoidal channels with trickle channels to convey normallow water flows. The maximum normal depth, Y,, is 5 feet and themaximum normal velocity, V,, is 7 fps.

b . Low Flow Channel: The low flow channel shown in this case is arectangular concrete channel. The concrete channel ends at theupstream end of the upstream apron. A combination cut-off walland foundation wall, to give the end of the low flow channel additionalsupport, is provided. The water is allowed to trickle through theupstream apron and through the vertical wall. low flow channelswould simply feather into the upstream apron.

C . Approach Apron: A lo-foot long apron is provided upstream of thecutoff wall to protect against the increasing velocities and turbulencewhich result as the water approaches the vertical drop. Heavyshould be used for this apron.

d . Crest Wall: The vertical wall should have the same trapezoidal shapeas the approach channel. The wall distributes the flow evenly over theentire width of the drop structure. This is important to prevent flowconcentrations which would adversely affect the basin.

The low flow channel is ended at the upstream end of the upstreamapron to prevent the low flows from concentrating additional water at apoint during high flows, thus exceeding the design assumptions. Theapron and the vertical wall combine to disperse the flow concentratedin the low flow channel. The low flows are allowed to trickle through the

, wall through a series of notches in order to prevent ponding. The voidsin the below the notch inverts are expected to silt in rapidly, or

they can be filled at the time of construction.



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The wall must be designed as a structural retaining wall. The top of thewall should be placed a distance P above the upstream channelbottom. This is done to create a higher water surface elevationupstream, thus reducing the effects normally caused by asudden drop. P can be determined from Table 1104.

e. Chute Apron: The stilling basin is designed to force the hydraulic jump to occur within the basin, and is designed for essentially zeroscour. The floor of the basin is depressed an amount B below thedownstream channel bottom, excluding the trickle channel. This isdone to create a deeper downstream sequent depth which helps keepthe hydraulic jump in the basin. This arrangement will cause pondingin the basin. The trickle channel can, depending on the depth, relieveall or some of the ponding. The can also be buried and vegetatedto reduce the area to a smaller size.

The basin is sized using Table 1103. To use the table, thedesigner must first determine the necessary height of the drop C, thenormal velocity of the approach channel V,, and the upstream anddownstream normal depths and Y,. Both channels must have thesame geometry and must be equal to Y,. Designs for drops when

shall be discussed with the local entity and/or the CCRFCDprior to design. Enter the row which contains the correct C, V,, and

and select the classification and necessary channel dropdimensions from that row.

The must be placed on bedding and filter fabric as shown in

Figure 1103. The should extend up the channel side slopes adistance of + 1 foot as projected from the downstream channel. Thebasin side slopes should be the same as those in the downstreamchannel (4: 1 or flatter) up to the + 1 foot location, above whichslopes as steep as are allowed.

f. Exit Depth: The downstream channel should be the same as theupstream channel, including a low flow channel. For concrete low flowchannels a foundation wall similar to the one used for the upstreamtrickle channel should be used. In some instances the wall may also beused to control seepage and piping.

A design example for a vertical drop structure is presented in Section1102.3.2.



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1102.1.3 Drops

drops are classified into three principal drop types according to theslope of their downstream face:

a . Vertical typeb . Stepped typeC. Sloped type

Presented in Figures 1104 through 1106 are examples of these three typesof drops.

are generally made in standard sizes ranging from 1 to 3 feet inthickness, 3 feet in width, and 6 to 12 feet in length. The number of cells ineach standard varies according to its dimensions, however, a cellshould not be greater than 3 linear feet. Manufacturers of should beconsulted and their design information should be reviewed before planningand designing a structure.

In constructing drops, either PVC coated wire mesh baskets orgalvanized steel wire mesh baskets are used. The designer shallobtain soil corrosion data for the specific site to determine which type ofcoating may be used or if the soil is too corrosive for either coating. The dataand coating recommendations shall be submitted to the local entity for reviewand approval.

Criteria:

The manufacturer suppling the should supply design guidelines andcriteria used in designing drops. Specific reference is made in thisMANUAL to MACCAFERRI, 1987. This reference outlines typical step by stepprocedures for design of drops and thus typical procedures are notrepeated in this MANUAL. The naming of MACCAFERRI should not beconstrued as an endorsement or acceptance of their products.

A few highlights of the design criteria are as follows:

Vertical Drops

1. Vertical drops are used for small drops.

2 . The maximum design discharge appears to be 100 cfs per foot.The recommended unit discharge is 35 cfs per foot.

3. The drop structure should be designed according to theprocedure specified in MACCAFERRI, 1987 or similarmanufacturers guidelines.



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Stepped Drops

1. Stepped drops shall only be used with light bed loads and witha maximum unit discharge of 35 cfs per foot.

2 . Technical reasons prevent in most instances a rational design ofstepped drops ensuring the formation of a hydraulic jump at thetoe of each fall. It is advisable not to rely upon energydissipation in each step.

3 . The drop structure should be designed according to theprocedure specified in MACCAFERRI, 1987 or similarmanufacturers guidelines.

Sloped Drop

1. Sloped drops are used where, due to the poor quality of thefoundation soil, a large foundation area and a fairly uniformpressure distribution are required.

2 . The maximum design discharge recommended is 35 cfs per foot.

3 . Design of the sloped drop should be based upon the proceduresspecified in MACCAFERRI, 1987 or similar manufacturersguidelines.

In addition. the following criteria should be considered in design of

drops:a . Where possible small drops in series would be more desired than one

large drop.

b . Counter weirs and stilling basins should be considered when scouringproblems are present. When they are not used the foundation of thedrop should be below the scouring ability of channel.

C. Structural stability should be checked for over turning and/or sliding.

d .In heavy debris areas the drop crest should be capped withconcrete to avoid debris blockage.

e. Design procedures may generally be obtained frommanufacturers free of charge or obligation.



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Section 1100 - Additional Hydraulic Structures

Adopted August 12, 1999 HYDROLOGIC CRITERIA AND DRAINAGE DESIGN MANUAL 1110

1102.1.4 Straight Drop Spillways

Presented in Figure 1107 are the design details for a straight drop spillway. Thespillway produces a controlled overflow jet which is dissipated through impact on

the structure floor and baffle blocks. The jet energy is also dissipated in theplunge pool created when impact blocks are used or through an hydraulic jumpusing the typical baffle block arrangements from the USBR stilling basin designs.

The basin design is based on the drop distance, Y, and the unit discharge, q, asrelated through the drop number, D, computed as follows:

D = q 2 / gY3 (1101)

where D = Drop Numberq = Unit Discharge (cfs / ft of width)

Y = Drop Distance (ft)

The remaining design parameters can be obtained from Figure 1107 .

The impact block basin is applicable for low heads with a wide range of tailwaterdepths. The hydraulic jump basin may be used as long as the design parametersfor the selected basin type are meet. The designer is referred to USBR, 1987,for detailed design information, guidelines, and examples.

1102.1.5 Baffled Aprons (USBR Type IX)

Presented on Figure 1108 is the baffled apron stilling basin. This structurerequires no initial tailwater to be effective, although when the tailwater forms apool into which the flow discharges, the channel bed scour is not as deep and isless extensive. The chutes are constructed on an excavated slope, 2:1 or flatter,extending to below the channel bottom. Backfill is placed over one or more rowsof baffles to restore the original streambed elevation. When scour or downstreamchannel degradation occurs, successive rows of baffle piers are exposed toprevent excessive acceleration of the flow entering the channel. If degradationdoes not occur the scour creates a stilling pool at the downstream end of thechute, stabilizing the scour pattern.

Generalized design information is presented in Figure 1109 . The designer isreferred to PETERKA, 1978 for detailed design information, guidelines, andexamples.

1102.2 Energy Dissipation Structures

Presented in Table 1101 are the types of energy dissipation structures allowedin the Clark County area. By definition, energy dissipation structures may be


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used for both sub-critical and super-critical upstream channel (or pipe) flowconditions. For sub-critical flow conditions, these structures are designed similarto the channel drop structures discussed in the previous section. For super-critical flow conditions, the upstream channel is tied directly into the stilling basinfloor (hydraulic rise) or the upstream channel is transitioned into the structurethrough the use of a trajectory transition section. The hydraulic design oftrajectory transition sections is discussed in Section 1102.2.9.

1102.2.1 Types of Energy Dissipation Structures

Many stilling basins and energy-dissipating devices have been designed inconjunction with spillways, outlet works, and canal structures, utilizing blocks, sills,or other roughness elements to impose exaggerated resistance to the flow. Thetype of stilling basin selected is based upon hydraulic requirements, availablespace and cost. The hydraulic jump which occurs in a stilling basin has distinctivecharacteristics depending on the energy of flow which must be dissipated in

relation to the depth of the flow. A comprehensive series of tests have beenperformed by the USBR for determining the most efficient energy dissipators(PETERKA, 1978).

The energy dissipation structures discussed herein provide a wide range ofstructures from which to choose the most hydraulically and cost efficient structure.The reader is encouraged to review the analysis, results, and recommendationin PETERKA, 1978, prior to final selection of energy dissipation structure.

1102.2.2 Stilling Basins With Horizontal Sloping Aprons

The basis for design of all of the USBR stilling basins is analysis of the hydraulic jump characteristics on horizontal and sloping aprons. The governing equationfor hydraulic jumps is based on pressure-momentum theory and may be writtenas follows:

D2 / D 1 = 0.5 ((1 + 8F 2r1)0.5 - 1) (1102)

where D 1 = Depth of Flow at Jump Entrance (ft)D2 = Depth of Flow at Jump Exit ft)F r1 = Froude Number at Jump Entrance

The results of the USBR analysis are presented in Figure 1109 . In this figure T wis the tailwater depth necessary to create or assist in forming the hydraulic jump.Generally, T w is greater than D 2.

The above equation is generally used to determine the approximate location ofa hydraulic jump in a channel. In practical application, the actual flow depths andlocation of the jump will vary due to inaccuracies in estimating actual flowparameters (i.e., channel roughness, flow characteristics). The location of the


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jump will also vary depending on the flow rate in the channel. Therefore, from astructural and safety standpoint, horizontal and sloping apron stilling basinsshould not be used as energy dissipation structures without the addition ofappurtenances (i.e., baffle blocks, end sills) to control the location of the hydraulic

jump. Standard designs for these types of structures are discussed in thefollowing sections.

1102.2.3 Short Stilling Basin (USBR Type lll)

Presented in Figure 1110 is the standard design for a Type lll stilling basin. Thechute blocks at the upstream end of a basin tend to corrugate the jet, lifting aportion of it from the floor to create a greater number of energy dissipatingeddies. These eddies result in a shorter length of jump than would be possiblewithout them, and tend to stabilize the jump. The baffle piers act as an impactdissipation device and the end sill is for scour control. The end sill has little or noeffect on the jump. The only purpose of the end sill in a stilling basin is to direct

the remaining bottom currents upward and away from the channel bed.

This type of basin is recommended at the outlet of a sloping channel drop whenthere is adequate tailwater. For insufficient tailwater, a USBR Type Vl basin isrecommended.

1102.2.4 Low Froude Number Basins (USBR Type IV)

Presented in Figure 1111 is the standard design of a low Froude number basin.The basin is used instead of the USBR Type ll and Type lll basins in order toachieve better jump characteristic at low Froude numbers (2.5 < F r < 4.5). Atthese low Froude numbers, excess waves are created because the jump is notfully developed.

This basin minimizes the waves by directing jets from the tops of the baffle blocksinto the roller to strengthen and intensify it. In addition, the tail water depth (T w)should be at least 1.1 x D 2 (conjugate depth) to minimize the chance of the jumpsweeping out of the basin (See Figure 1109 ). The end sill has little or no effecton the jump but rather directs the bottom currents upward and away from thechannel bed.

1102.2.5 Impact Stilling Basin (USBR Type Vl)

This stilling basin is an impact-type energy dissipator, contained in a relativelysmall box-like structure, and requiring little or no tailwater for successfulperformance. The general arrangement of the basin is shown on Figure 1112 .This type of basin is subjected to large dynamic forces and turbulences whichmust be considered in the structural design. The structure should be madesufficiently stable to resist sliding against the impact load on the baffle wall andmust resist the severe vibrations. Riprap should also be provided along the


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Energy dissipation structures may be designed for either sub-critical or super-critical upstream flow conditions. For sub-critical flow, an abrupt change in gradeat the structure entrance performs satisfactorily. However, for super-critical flow,the flow tends to separate and spring away at any abrupt change in grade.Therefore, to avoid the possibility of flow separation from the channel floor, thefloor shape should be flatter than the trajectory of a free discharging flow jet.

Presented in Figure 1113 is a typical design of a trajectory transition section.The curvature of the trajectory section can be determined by the followingequation.

y = x tan 2 + x2 / K (4(d + h v) cos 2 2) (1103)

where Y = Change in Vertical Elevation (ft)X = Change in Horizontal Location (ft)K = Safety Factor

d = Depth of Flow at Trajectory Entrance (ft)hv = Velocity Head at Trajectory Entrance (ft)2 = Slope Angle From Horizontal of the Upstream Channel (Degrees)

The safety factory, K, should be equal to or greater than 1.5 to assure positivecontact pressure.

The trajectory section should be connected to the stilling basin apron by a short,steep chute section. This section should be at a slope between 1.5 horizontal to1 vertical and 3 horizontal to 1 vertical with 2 horizontal to 1 vertical preferred. Inno case should the slope be flatter than 6 horizontal to 1 vertical.

1102.3 Exa mp le Ap plic ations

The following example applications present typical design calculations for variouschannel drop and energy dissipation structures. The reader is referred toPETERKA, 1978, for additional design examples.

1102.3.1 Example: Sloping Riprap Drop Structure

Problem: Design a sloping riprap drop for a channel with the followingcharacteristics:

Q = 1,600 cfs

Upstream and Downstream Channel ParametersBottom Width = 50 ft S = 0.0043 ft / ftSide Slopes = 4:1 Y c = 2.9 ftYn = 4.0 ft V n = 6.0 fpsConcrete Low Flow Channel


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Drop Required = 3.0 ft

Solution:

Step 1: Determine Maximum Unit Discharge.

q = V n Yn = (6.0 fps) (4 ft) = 24 cfs / ft

Step 2: Select the chute slope from Table 1102 for q = 25 cfs / ft

The following options are available:

1) Heavy Riprap 10:1 or Flatter; DR = 1.75 ft, DRW = 2.6 ft

2) Grouted Riprap at 6:1 or Flatter; DR = 2.6 ft; DRW = 3.25 ft

The best slope will depend on factors such as availability and costof the riprap bedding and filter cloth and ROW limitations. For thisexample, a 7:1 slope was selected.

Step 3: Select Length of Downstream Apron L B = 20 ft

Step 4: Determine Crest Wall Elevation. ( Table 1104 )Bottom width = 50 ft, Y n = 4.0 ftUse P = 0.1 ft

1102.3.2 Example: Vertical Riprap Drop Structure

Problem: Design a vertical riprap drop for a channel with the followingcharacteristics:

Q = 1,600 cfs

Upstream and Downstream Channel Parameters:Bottom Width = 50 ft S = 0.0043 ft / ftSide Slopes = 4: 1 Y c = 2.9 ftYn = 4.0 ft V n = 6.0 fpsConcrete Low Flow ChannelDrop Required = 3.0 ft

Solution:

Step 1: From Table 1103 , for C = 3.0 ft, V n = 6.0 fps and Y n and Y 2 =4.0 ft


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Select the riprap designation and the riprap basin dimensions.

Riprap - Heavy

B = 1.0 ftA = 2.5 ftLB = 20 ftD = 5.0 ftE = 4.0 ft

Step 2: Determine P = 0.1 from Table 1104

Step 3: Design retaining wall and finalize dimension

1102.3.3 Example: Vertical Gabion Drop Structure

Problem: Design a vertical gabion drop structure for a channel with thefollowing characteristics:

Q = 1,600 cfs

Upstream and downstream channel parameters are:

Bottom Width = 60 ft S = 0.0043 ft / ftSide Slope 4:1, Y c = 2.63 ftYn = 3.3 ftDrop Required = 4 ft

Assume the drop is to be built with a lined stilling pool floor andcounter drop (weir).

Refer to Figure 1114 for definition of variables.

Solution:

Step 1: Design of Crest:

Assume the width of the rectangular weir, Lg = 48 feet and lengthalong stream = 12 feet minimum, (use more if needed for stability).Assume C = 3.1 in the weir formula:

Q = CL g (Zo -fg) 3/2

Zo -fg = Q2/3 / CLg = 1,600 2/3 / 3.1 x 48

Use 6 feet as the height of the crest.


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Step 2: Design of Stilling Pool:

Assume the width of the counter drop = 54 feet

q = 1,600 / 54 = 19.6 cfs / ft

With q = 29.6 cfs and Z o -fb = (4 + 4.9) = 8.9 ft using Figure 53 inMACCAFERRI,1987

Z1 -fb = 1.25 ft

With Z 1-fb = 1.25 ft and q = 29.6 and using dashed line in Figure 53in MACCAFERRI, 1987

Z2 -fb = 5.8 ft

Determine Z 2 -fc by using weir equation

Z2 -fc = Q2/3 / CLb = (1,600) 2/3 / 3.1 x54

The height of the counter drop is

fc -fb (Z2 -fb) - (Z2 -fc) = 5.8 - 4.5 = 1.3 ft

Use 6 feet (minimum) as the length of the counter drop alongstream. Determine the length of the stilling pool, L b

Lb = Lg1 + L12

Since the drop is backed by streambed material

Lg1 / (fg -fb) = 4.30D 0.27 (Eq.16)

(Eq. 15) D = q 2 / g (f g -fb)3

D = (1,600 / 48) 2 / 32.2 (4) 3

Lg1 = 4.30 (0.5392) 0.27 x 4 = 14.6 ftL12 = 6.9 (Z 2 - Z1) (Eq. 20)= 6.9 x [ (Z 2 -fb) - (Z1 -fb) ]= 6.9 x (5.8 - 1.25)= 31.4 ftLb = Lg1 + L12 = 14.6 + 31.4 = 46 ft

Use L b = 48 ft


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Note: The above Equations 15, 16, and 20 are found inMACCAFERRI, 1987.

Step 3: Check for seepage:

The total path L of seepage under and the structure must be L > C) H (Equation 22, MACCAFERRI,1987)

Where C is a coefficient depending on the type of soil, and ) H isthe difference between the upstream and downstream watersurfaces.

Assume C = 6 (riverbed sediment)Therefore L > 6 x 5.5 = 33 ft

The length of stilling pool itself is 48 ft > 33 ft. Thus seepage

consideration is satisfied.

To prevent undermining of the counter drop (weir), an apron isconstructed downstream; the length of this apron will beapproximately 9 feet.

Step 4: To complete the design of the drop structure, a stability analysiswill be required to determine the required thickness of the gabionmattresses and depth to foundation. This stability analysis isbeyond the scope of this manual. The user should refer toMACCAFERRI, 1987 for guidance for stability analysis.

The hydraulic design dimensions of the structure are given inFigure 1114 .

1102.3.4 Example: Impact Stilling Basin

Problem: Design an impact stilling basin (USBR Type Vl) for a 48-inch RCPoutlet with the following parameters:

Pipe Dia = 48 in RCPQ = 214 cfsV = 17 fps < 30 fps (upper limit)Tail water depth = 2.5 ftChannel slope = 1.0 percent


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Solution:

Step 1: Using the discharge (Q = 214 cfs) enter the discharge limitsportion of Figure 1109 and read the maximum and minimum basinwidth.

Wmin = 12.5 ftWmin = 15.0 ft

Step 2: From the basic dimension portion of Figure 1109 and using thedischarge Q = 214 cfs, interpolate for the basin dimensions. Notethat the corresponding pipe size in the table is between a 54-inchand 60-inch diameter, which is larger than the example pipe sizeof 48 inches. The basin will therefore provide ample room for theexample pipe.

The basic basin dimensions are as follows:

W = 12 ft - 4 in b = 10 ft - 6 inH = 10 ft - 3 in c = 5 ft - 8 inL = 18 ft - 2 in d = 2 ft - 4 inA = 7 ft - 8 in g = 5 ft -1 in

Step 3: Determine length of downstream riprap.

Downstream length = W = 12 ft - 4 inUse a downstream length of 13 in

Step 4: Determine size and thickness of downstream riprap.

For a basin width of 12 ft - 4 in, use a 3 ft layer of heavy riprap perSection 1102.2.7.


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Additional Hydraulic Structures

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